1. Field of the Invention
The present invention relates to a method and device for solvent recovery in an ultrasonic cleaning machine, which method and device can effectively liquify and recover solvent vapors.
2. Description of the Prior Art
Conventional solvent recovery methods will be explained with reference to FIG. 2(a) and FIG. 2(b).
FIG. 2(a) is an explanatory drawing relating to a first conventional solvent recovery method. The system used comprises a distillation vessel DV, a heater H, a freeboard FB which forms a channel d in the outer peripheral edge of the distillation vessel DV, a cooling coil C attached to the inner peripheral wall of the freeboard FB and connected to a refrigerating machine (omitted from the drawing), and a solvent R.
The solvent R heated by the heater H is gradually converted to vapor and evaporates upward to be cooled by the cooling coil C and liquified. The liquid solvent then drips down into the channel d to be recovered. At the same time that the solvent vapor is liquified to give the recovered solvent R, the water vapor in the air is liquified and separated out in a moisture separator (not shown in the drawing) before the solvent R is returned to the specified solvent vessel.
FIG. 2(b) is an explanatory drawing relating to a second conventional solvent recovery method. The members which perform the same functions as those in FIG. 2(a) carry the same code numbers.
This second method is usually used for the solvent freon which ceases to give off vapor at -15.degree. C. The cooling coil C is duplicated to cool the solvent vapor as far as possible to -15.degree. C. to prevent the vapor from escaping from the vessel. Otherwise the method is identical to the first conventional method in which the vapor is cooled by the respective cooling coil C and liquified, after which the liquid drips down into the channel d from which the solvent R is recovered and returned to the specified solvent vessel via the moisture separator.
In both the first and second conventional methods the solvent vapor is cooled and liquified by the cooling coil C which is positioned at the peripheral edge of the vapor zone. To prevent the solvent vapor from escaping from the solvent vessel, the height of the normal freeboard FB must be 1.5 times the width of the open section. This gives rise to the drawback that the device is generally very high. In particular, although the second conventional method is comparatively effective in preventing the escape of the solvent vapor, the device has the cooling coil built up in two stages, which unavoidablely enlarges the longitudinal dimension of the device.
In addition, the relationship between the amount of solvent vapor generated and the amount of solvent vapor cooled and liquified by the cooling coil C is occasionally subjected to momentary changes related to the intermittent operation of the refrigerating machine (omitted from the drawings), to changes in the amount of heating by the heater H, to changes in the ambient temperature, and to changes in other conditions. Accordingly, when the amount of solvent vapor generated is small compared to the amount of solvent vapor cooled and liquified, specifically, when the height of the solvent distillation zone is small, the cooling coil C is exposed to the air in the upper zone of the solvent vapor, and, because the moisture in the air is condensed, the ratio of moisture mixed into the recovered solvent is high. This results in the drawback of unsatisfactory cleaning in the ultrasonic cleaner because of the water present. This problem is particularly remarkable in the second conventional method in which the cooling and liquifying capacity is increased. On the other hand, if the amount of solvent vapor generated is larger than the amount of solvent vapor cooled and liquified, the height of the solvent vapor zone is greater than the height of the cooling coil C. In such a case the solvent vapor escapes from the vessel so that it is difficult to completely recover the solvent vapor.